This invention relates generally to human hearing, and more specifically to the design and surgical insertion and positioning of an implantable microphone.
It is estimated that some form of hearing impairment affects over 7% of the U.S. population. Such hearing impairment can be caused by a myriad of factors, for example, trauma, ear infections, congenital factors, ototoxic effects from some antibiotics, and from diseases such as meningitis.
Mild forms of hearing impairment can generally be aided by use of conventional BTE (behind the ear) or CIC (completely in the canal) type hearing aids. Severe hearing impairment may be ameliorated by use of high power conventional hearing aids. For profound hearing loss, the use of cochlear implants may be the only alternative. Other specialized hearing aids, such as bone condition devices, are also available for certain types of hearing impairment.
Conventional hearing aids function by simply amplifying the acoustic signal and transmitting such amplified signal to the ear canal. However, there appears to be a significant social stigma to wearing conventional hearing aids. Also, conventional hearing aids have problems with moisture, audio feedback, ear wax buildup, irritation of the auditory canal skin, amplification of unwanted background noise, and non-linear acoustic distortion, especially at high amplification. Thus, for aesthetic and technical reasons, considerable effort has been directed to developing partially or totally implantable hearing devices. Such devices generally involve one of three basic technologies, namely, vibration of one of the ossicular bones in the middle ear, vibration of the skull bone (i.e. bone anchored devices), or cochlear implants. Prior art devices were usually partially implanted, where part of the electronics, including the microphone, was positioned outside the body near the ear. A review of this technology is presented in xe2x80x9cCurrent Status and Critical Reflections on Implantable Hearing Aidsxe2x80x9d by K. B. Huttenbrink, Amer. J. of Otology, 20:409-415, 1999. Two of the key technical limitations to achieving totally implanted hearing devices are the existence of a suitable implantable battery and the availability of an implantable microphone. However, some groups have now developed totally implantable hearing devices, some of which are undergoing clinical trials. A brief review of these developments is given by M. Chasin in Hearing Health, Vol. 15, No. 3, May/June 1999, pages 40-41 and 47.
Since totally implantable hearing aids or cochlear prostheses require some form of implantable microphone, there exists prior art dedicated to fabricating a functional implantable microphone. For example Mahoney in U.S. Pat. Nos. 3,346,704 and 3,557,775 teaches a simple silicone rubber tube (approx. 15 mm long) sealed at both ends with a very thin silicone membrane, connected to a microphone at one end. The tube is extended externally from the antrum cell of the mastoid and disposed just beneath the skin, with vibrations picked up through the skin by said tube. T. Ohno et al. pp. 67-68 in Advances in Audiology, Vol. 4, 1988, edited by M. Hoke, describe an electret microphone encased in a stainless steel housing, designed to be located in the wall of the external auditory canal. However, their design of the microphone housing was relatively large and impractical for such implantation. Hortmann et al. in U.S. Pat. No. 5,411,467 teach an implantable microphone connected to a sound conducting tube which distal end is closed by a membrane and projected into the tympanic cavity for acoustic pickup. Money in U.S. Pat. No. 5,782,744 describes a microphone which uses the pressure fluctuations generated within the perilymph fluid of the cochlea scalae as sensing means. Muller et al. in U.S. Pat. No. 5,814,095 describe an electret microphone encased in a titanium housing, which housing has two legs which are oriented at an angle relative to one another, where one leg holds the microphone capsule and covering diaphragm, and the other leg contains the electrical connectors. Leysieffer et al. in U.S. Pat. No. 5,999,632 describe the addition of a projecting elastic flange (on the skin side of the wall of the auditory canal) and another flange near the elbow joint of the two-legged device. Ball et al. in U.S. Pat. No. 5,859,916 describe a two stage implantable microphone where an electret microphone is contained in an internal chamber, which chamber is coupled to another chamber covered with a thin diaphragm. Said diaphragm is protected with a cover containing holes. Lesinski et al. in U.S. Pat. No. 5,881,158 depict a relatively large diameter (10 mm) microphone positioned just below the skin behind the external ear. Their microphone uses a metalized electret (Teflon) film over a partially supported substrate that allows said film to flex due to an acoustic signal. Jaeger et al. in U.S. Pat. No. 5,888,187 teach an implantable two-stage microphone for use by vocally impaired persons. This device is very similar that that shown in U.S. Pat. No. 5,859,916.
Anatomically, there are a variety of possible locations for implanting a microphone. For example, the microphone can be positioned in the bony or cartilaginous ear canal, on the surface of the temporal bone either behind (posterior) or in front (anterior) of the ear, or on either the medial or lateral side of the pinna. Also, a smaller microphone could be inserted into the pinna by attaching it to the cartilage of the pinna or the underlying adjacent bone, with a window made in the pinna cartilage for better acoustic coupling to the microphone input. However, according to the present invention the microphone is anchored (via osseointegration to the bone) in the posterior wall of the bony ear canal. This presents a number of major advantages since sound entering the auditory canal creates a tuned acoustic resonator whereby some key voicing frequencies are enhanced (see for example, A. E. Deddens, et. al., Am. J. Otolaryngol, 11:1-4, 1990; and J. A. Feigin, et. al., Ear and Hearing, Vol. 11, No. 5, 1990). Also, the skin lining the auditory canal is very thin (about 0.1 to 0.2 mm), thus allowing the acoustic signal to be easily transmitted through said skin, thereby inducing relatively unmodified acoustic vibrations in the microphone membrane. These membrane vibrations are sensed by the encapsulated electret microphone.
Other advantages of locating the microphone in posterior wall of the ear canal include: (a) the implantee hears more naturally via the concha and pinna, (b) the bony canal is a good mechanical structure in which to anchor and osseointegrate the microphone housing, thereby minimizing housing migration post surgery and (c) the microphone is safely located and not easily damaged by blows to the side of the head.
It is an object of this invention to provide a totally implantable microphone which provides good acoustic response, is securely retained in the bony ear canal, is adapted to resist high and low ambient pressures, which can be implanted with relative ease and which is of small dimensions to minimize bone dissection and interference with the sigmoid sinus and other adjacent structures.
The apparatus according to the invention comprises a hermetically sealed small commercial electret type microphone in a biocompatible cylindrical housing, which housing contains novel features adapted for effective, safe and long-term implantation in the posterior wall of the auditory canal.
One feature of the invention is the means by which the thin metallic membrane covering the acoustic input (anterior) end of the microphone housing is protected from rupture during surgical handling, or during subsequent exposure by the implantee to high pressure (i.e. during diving) or to low pressure (i.e. high altitude). Said inventive means has the collateral advantage of greatly increasing the sensitivity of the encapsulated electret microphone to measure the acoustic signal in the air canal. An additional aspect of the invention is the addition of a posteriorly disposed ring-shaped flange on the body of the housing, such flange acting to prevent the microphone housing from migrating anteriorly into the ear canal, thereby rupturing the thin skin lining the wall of the exterior auditory canal or actually extruding.
The invention is a totally implantable microphone, suitable for use with an implanted hearing prosthesis. Said invention comprises a hermetically sealed electret type microphone encapsulated in a cylindrical bio-compatible, preferably metallic, housing, with a flat bio-inert, preferably metallic, base closing one end of said housing, a plurality of electrically insulated lead-throughs disposed through said base, a substantially flat bio-inert metallic membrane closing the other end of said housing, and a support plate positioned below said membrane, such support plate acting to limit the movement of the overlaying membrane during inward flexure of said membrane. In one embodiment of the invention, said membrane contains at least one externally (or internally) protruding ridge near the outside perimeter of said membrane, such ridge acting to provide stress relief for the outward bulging of said membrane during conditions where the pressure outside the housing is lower than the pressure inside the housing.
In a further embodiment of the invention, one or more ridges and or grooves are radially or spirally disposed along the length of the outside wall of said bio-inert housing, so as to assist said housing wall to osseointegrate with the bony wall of the ear canal. Also, a continuous spiral groove (i.e. similar to a screw thread) could be used on the housing outside wall to promote ossiointegration with ear canal bone.
In a yet further embodiment of the invention, lithographically formed wires are laminated in a thin inert polymer, where said wires are connected to the electrical lead-throughs on the flange-side of said housing end, creating a very compact hermetic electrically insulated lead-through configuration, making the overall housing length short, thereby alleviating surgical issues regarding positioning the microphone housing a safe distance from the sigmoid sinus. Said electrically insulated lead-throughs can be achieved by using a base, preferably made of titanium, containing ceramic or bio-inert glass insert(s) acting as electrical insulators, said ceramic (or glass) insert sealed to the titanium base with bio-inert metal (or metals) such as, niobium and/or gold, and, in a further embodiment, the electrical lead-throughs through the glass or ceramic inserts use a bio-inert conductive metal such as gold, tin or an alloy thereof, to create an electrical contact between the microphone and the lithographically formed wires laminated in a thin bio-inert polymer.
Any type of microphone can be encapsulated in the hermetically sealed housing, however, the preferred embodiment is to use a small electret type microphone.
In one aspect of the invention, a totally implantable microphone comprises a microphone encapsulated in a cylindrical bio-inert metallic housing. A bio-inert base is provided at one end of said housing and a membrane closes the other end of the housing. A plurality of electrically insulated lead-throughs are disposed through the base.
In another aspect, a support plate is positioned below the membrane to support the overlaying membrane during inward flexure.
In another aspect, the membrane includes one or more ridges near its outer perimeter to provide stress relief for the outward bulging of the membrane when the pressure outside the housing is lower than the pressure inside the housing.
Preferably one or more ridges and or grooves are radially or spirally disposed along the length of the outside wall of the housing to assist with osseointegration with the bony wall of the ear canal.
In another aspect, a flange is provided on the posterior end of the housing to prevent post-operative migration of the housing into the auditory canal.
In yet another aspect, lithographically formed wires laminated in a thin inert polymer are connected to the electrical lead-throughs on the flange-side of the housing end. This creates a compact hermetic electrically insulated lead-through configuration and makes the overall housing length short.
Preferably the metallic housing and membrane are both made of titanium and the metallic base is comprised of titanium containing one or more ceramic or bio-glass inserts acting as electrical insulators. The insulators are sealed to the titanium base with a low melting point metal or metal alloy.
In yet another of its aspects, the invention is a totally implantable microphone comprising a microphone encapsulated in a cylindrical bio-inert housing, a bio-inert base at one end of the housing and a plurality of electrically insulated lead-throughs disposed through the base. A membrane closes the other end of the housing and a support plate is positioned below the membrane and acts to support the overlaying membrane during inward flexure of the membrane. At least one ridge is provided in the membrane near its outside perimeter to provide stress relief for the outward bulging. A peripheral flange is provided on the posterior end of the housing and lithographically formed wires laminated in a thin inert polymer are connected to the electrical lead-throughs on the flange-side of the housing end.
In yet another aspect the invention comprises the use of a microphone as described above in whereby it is implanted in the wall of the auditory canal such that the membrane underlies the skin of the ear canal and the base is substantially flush with the outside of the wall of the auditory canal.
In another aspect the invention is a microphone assembly for use with an auditory prosthesis comprising a microphone having a cylindrical housing, the microphone being implanted in the wall of the auditory canal such that the membrane underlies the skin of the ear canal and the base is substantially flush with the outside of the wall of the auditory canal. An electronics package is retained in the vicinity of the mastoid cavity, wires are laminated in an inert polymer film and extend from the microphone to the electronics package and a connector line extends between the electronics package and an auditory prosthesis.
Other aspects of the invention will be appreciated by reference to the following descriptions and to the claims.